"I've photographed more than 3,000 cats so far [at shelters] and no two are the same," Marjan Debevere who is a cat shelter photographer in London told BBC. "I've witnessed a lot of cats purring when they're dying, and when they're being put to sleep. The vet will say something like 'They were purring right up until the end', and people assume they're happy when they're purring. That's just not always the case."
In the early 2000s, experts hypothesised that purring has other purposes besides this and in recent years more light has been shed on the purr.
"Researchers have recorded 'ordinary purrs' and purrs that were soliciting food from their owners," says Celia Haddon, an author and cat behavioural expert. "Even non-cat owners could tell the difference. Inside the ordinary low purr was a higher frequency cry, somewhat like a meow."
"It's likely that purring has communication, appeasement, and healing properties," says Gary Weitzman, a veterinarian and CEO of the San Diego Humane Society. "While the purr does generally represent contentment for cats, it can also express nervousness, fear and stress. Fortunately, more often it's an indicator of the former."
"It calms us and pleases us, like watching waves against a beach. We respond to a cat's purr as a calming stimulus," says Weitzman.